1. Field of the Invention
The present invention relates to a Multi-Input Multi-Output (MIMO) communication system, and more particularly, to a method for transmitting and receiving a precoded signal in a MIMO communication system.
2. Discussion of the Related Art
Recently, demand for wireless communication services has rapidly increased due to the spread of information communication services, the introduction of a variety of multimedia services, and the advent of high-quality services. To actively cope with such demand, first of all, it is necessary to increase the capacity of the communication system. Exemplary methods of increasing the communication capacity in wireless communication environments include a method of discovering a new available frequency band and a method of increasing the efficiency of limited resources. As a method for increasing the efficiency of limited resources, a so-called Multi-Input Multi-Output (MIMO) communication technology, in which multiple antennas are installed on a transmitter/receiver to secure an additional spatial area for utilization of resources, thereby achieving a diversity gain, and data is transmitted in parallel through the multiple antennas to increase transmission capacity, has recently been intensively developed while attracting a lot of attention.
The MIMO communication system requires a variety of technologies to increase the reliability of data transmission. Among these technologies, a preceding scheme is used to increase a Signal to Noise Ratio (SNR). Examples of the preceding scheme include a codebook-based preceding scheme, which is used when feedback information is limited in a closed-loop system, and a scheme in which channel information is quantized and fed back. In the codebook-based preceding, the receiving end feeds an index of a preceding matrix, which is already known to both the receiving end and the transmitting end, back to the transmitting end to achieve an SNR gain.
Since feedback of all channel information causes significant overhead, a MIMO preceding scheme, which uses partial channel information, has been standardized and applied to actual systems. In the MIMO precoding scheme, channel information is quantized to construct a codebook and an index allocated to each codebook component is transmitted through a control channel.
FIG. 1 illustrates example configurations of transmitting and receiving ends of a MIMO communication system which uses codebook-based precoding.
In FIG. 1, each of the transmitting and receiving ends contains a limited number of preceding matrices P1 to PL, the receiving end feeds an optimal feedback index (i.e., an optimal preceding matrix index 1) back to the transmitting end using channel information, and the transmitting end applies a preceding matrix corresponding to the fed-back index to transmission data (X1 to XMt).
The MIMO preceding scheme is a closed-loop MIMO scheme in which part or all of channel information is fed back to a base station or a terminal to achieve improvement of communication performance. This scheme exhibits advantageous effects, especially in a slow fading environment of 30 Km or less per hour.
For reference, the following Table 1 illustrates an example codebook that can be applied when 3-bit feedback information is used in an IEEE 802.16e system which has 2 transmit antennas and which supports a spatial multiplexing rate of 2.
TABLE 1MatrixIndex(binary)Column 1Column 200010010010.7940−0.5801 − j0.1818  −0.5801 + j0.1818  −0.79400100.79400.0579 − j0.60510.0579 + j0.6051−0.79400110.7941−0.2978 + j0.5298  −0.2978 − j0.5298  −0.79411000.79410.6038 − j0.06890.6038 + j0.0689−0.79411010.32890.6614 − j0.67400.6614 + j0.6740−0.32891100.51120.4754 + j0.71600.4754 − j0.7160−0.51121110.3289−0.8779 + j0.3481  −0.8779 − j0.3481  −0.3289
On the other hand, technologies such as Per Antenna Rate Control (PARC), Per Stream Rate Control (PSRC), and Per User Unitary Rate Control (PU2RC) suggested in the 3GPP Long Term Evolution (LTE) standard can also be implemented in a MIMO system structure. In 3GPP LTE, a preceding scheme has also been introduced as a closed-loop MIMO system scheme. Examples of the preceding scheme include PU2RC and SIC-based Per User and Stream Rate Control (S-PUSRC).
In the case of the PU2RC scheme, matrices extended according to the number of transmit antennas using the Fourier transform are used as unitary matrices for preceding as shown in the following Mathematical Expression 1.
                                          e            m                          (              g              )                                =                                                    1                                  M                                            ⁡                              [                                                      w                                          0                      ⁢                      m                                                              (                      g                      )                                                        ,                  F                  ,                                      w                                                                  (                                                  M                          -                          1                                                )                                            ⁢                      m                                                              (                      g                      )                                                                      ]                                      T                          ⁢                                  ⁢                              w            nm                          (              g              )                                =                      exp            ⁢                          {                              j                ⁢                                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    n                                    M                                ⁢                                  (                                      m                    +                                          g                      G                                                        )                                            }                                                          [                  Mathematical          ⁢                                          ⁢          Expression          ⁢                                          ⁢          1                ]            
In Mathematical Expression 1, em(g) is a unitary preceding vector, “M” denotes the total number of antennas, and “G” denotes the total number of groups of preceding matrices. In addition, “n” and “g” denote an nth antenna and a gth group, respectively. A preceding matrix can be specified using the numbers “n” and “g.” Also, “m” denotes an mth virtual beamforming pattern.
In the case of S-PUSRC, a switching beamforming vector is used as a preceding matrix as shown in the following Mathematical Expression 1.P=[a1,a2Fa2N]ai=[1,ejφi,F,ej(N−1)φi],φi=kd sin(θi)  [Mathematical Expression 2]
where “N” denotes the number of antenna elements, “ai” denotes a preceding vector, “k” denotes a wavelength, “θ” denotes a steering direction, and “d” denotes the distance between neighboring antenna elements.
In the case of single-user MIMO of the closed-loop system, accuracy varies according to the amount of data of an antenna weight fed back from a terminal and thus MIMO performance depends on the amount of such feedback data. Especially, when the number of transmit antennas is 4, the size of the corresponding codebook is large and therefore the amount of feedback data is also large.
In addition, reception performance is significantly affected by how a codebook is designed. Accordingly, it is important to design a codebook which exhibits high performance while reducing the amount of feedback data. Another important factor when a MIMO codebook is designed is the complexity of the receiver which receives the codebook.
Although conventional codebooks exhibit excellent performance for channels with a low degree of correlation, they have been designed such that they have difficulty achieving a performance improvement when the degree of channel correlation is high. Thus, the conventional codebooks tend to exhibit different performances depending on the structure and interval of antennas. The conventional codebooks also have problems in that it is not easy to extend the codebook size and it is also difficult to design a systematic codebook having adaptability according to the channel status (for example, according to the rank).